The Kensington mine is on hold until the decision, which the court has until June to release. The mine's permits were invalidated after the 9th Circuit Court of Appeals agreed with three environmental conservation groups that the permits violated the act. The groups that oppose dumping mine tailings are the Southeast Alaska Conservation Council, the Juneau Group of the Sierra Club and Lynn Canal Conservation.

"We're preparing to move forward with construction of the tailings dam upon a favorable decision," said spokesman Tony Ebersole of Coeur d'Alene Mines Corp., parent company of mine operator Coeur Alaska Inc., after the arguments. "Our actions in terms of what we've been doing up to this point speaks to our commitment to Kensington."

But the company won't say what happens if it loses. Last year, Coeur was applying for a permit for an upland tailings facility, which environmentalists liked. It pulled out of that permitting process in September, and now is relying on a favorable decision from the court.

Until Coeur cancelled that process, environmentalists and mine officials had both touted their discussions, but whether they will keep talking is uncertain. Coeur met with environmentalists in early December but no agreements were made, said Rob Cadmus, water quality and mining organizer for the Southeast Alaska Conservation Council.

About 40 workers are now employed, but the mine could employ about 300 when in production.

Juneauites at the court Monday included Randy Wanamaker, who has worked to ensure Coeur hires Alaska Natives.

"This is employment for people who need jobs," he said. "All the environmental issues have been properly addressed."

Fill vs. discharge

Environmental groups sued over how the federal government should regulate the mine's tailings, or the ground-up waste rock that's left over after metals are removed.

Is it wastewater discharge from the ore-removal process, or is it fill?

If it's discharge, it can't go in the 23-acre lake. The U.S. Environmental Protection Agency has a rule that prohibits depositing wastewater from the gold milling process into U.S. waters.

But if it's fill, it's subject to less strict regulation by the U.S. Army Corps of Engineers and will be allowed in the lake, which is how the Corps treated it.

Earthjustice attorney Tom Waldo, representing the environmental groups, says tailings are both. They're fill because of all the solid material, but that doesn't exempt them from the effluent limits.

The federal government and Coeur, supported by the state of Alaska, say it's fill, period.

The government also argues that the agencies know best how to interpret their own laws, and the court ought to defer to them.

A lively debate

The justices' questions went into the weeds of how the regulations work, as well as the practical context of the mine's alternatives, or what would happen to Lower Slate Lake and other lakes.